Demand is driving semiconductor manufacturers to make the overall size of semiconductor devices smaller and smaller. One method of reducing device size is to assemble a device in a protective body or package which utilizes tape automated bonding (TAB) leads. TAB leads are formed from a very thin conductor which is laminated to a film carrier rather than from a stamped or etched metal frame used to form other conventional leads. For this reason, TAB leads can be made much smaller, resulting in smaller semiconductor devices. Usually, TAB leads are formed of a copper laminated layer on one or both sides of the carrier film. Copper is laminated on the film in order to provide conductive paths or traces to form leads. The copper leads are often plated with gold for better adhesion as will be evident below.
In a semiconductor device, TAB leads are bonded to a set of bonding pads formed on a semiconductor die in order to provide electrical connection to various circuits on the die. The fragile nature of TAB leads and clearance requirements needed to make reliable bonds prohibit TAB leads from being directly bonded to most bonding pads. In order to have suitable TAB lead bonding, an intermediate bonding feature, commonly referred to as a bump, is usually formed on each of the bonding pads.
Bumps are most often comprises of gold to enhance lead adhesion and are formed on bonding pads of a semiconductor die by one of several ways. One bumping technique is plating. In the bump plating process, an entire surface of the die having the bonding pads located thereon is initially coated with a thin layer of sputter deposited gold. A mask, for example photoresist, is formed on the thin gold layer and is patterned to expose the sputtered gold layer overlying the bonding pads of the die. The exposed portions of the sputtered gold layer are then plated with gold, usually by electrodeposition, to increase the gold thickness over the bonding pad area, thereby forming a plurality of gold bumps on the die surface. After forming the bumps, the mask is removed and a quick chemical etch is used to remove the sputter deposited gold layer on the non-bumped portions of the die surface. The sputter deposited gold layer is used only to provide a surface which can be readily gold plated and is therefore removed after the bumps are formed to prevent the bumps from being short-circuited to one another.
While gold plated bumps establish reliable bonds to TAB leads, the use of gold plated bumps has several disadvantages. A significant disadvantage is the prohibitive cost associated with the plating process. Plating requires all die on a given wafer to be bumped, regardless of whether or not the die is functional. For example, on a wafer which yields 85 percent functional or "good" die, 15 percent of the die will be bumped unnecessarily, thereby adding cost to the functional die. Furthermore, plating requires a series of steps including sputtering, masking, plating, and etching, each of which increases fabrication costs. Yet another disadvantage with plated bumps is that the series of fabrication steps increases fabrication time which in turn also increases cost.
Another bumping technique which overcomes many of the above disadvantages is ball bumping. Ball bumping referes to a process which utilizes a conventional wire bonding tool to form bumps on bonding pads of a semiconductor die. Semiconductor devices which do not employ TAB leads often have metal leads which are coupled to bonding pads by a very fine gold wire. One end of the wire is attached to the bonding pad while another end is attached to a tip of the lead which is usually spaced apart from the die. In the ball bumping technique, the fine gold wire is modified so that the wire does not span from the die to a lead. Instead, the wire is bonded to the bonding pad and then cut or severed. In bonding the wire to the bonding pad, the wire is compressed into a ball shape, thus the term "ball bump". FIG. 1A illustrates an example of a known semiconductor device 10 which utilizes ball bumps. The device includes a semiconductor die 12 having a plurality of bonding pads 14 located on one of the die surfaces. A ball bump 16, formed from a fine gold wire, is bonded to each of the plurality of bonding pads. Ball bumps 16 are bonded to the bonding pads by compressing the wire against the bonding pads and then severing the wire to form the bump. Upon bonding the wire, a rounded base portion 17 of the bump is formed as a result of compressing the wire against the die surface. A tail portion 19 above base portion 17 is also formed as the wire is drawn away from the die surface during the bonding procedure. At a predetermined distance from the die, the wire is broken to complete the ball bump. As a cross-sectional view in FIG. 1B illustrates, it is difficult to bond a lead 13 to a ball bump 16 due to the shape of the bump. Upon bonding lead 13 to the bump, tail portion 17 deflects the lead, for example as indicated by either of dashed leads 13' and 13", causing substantial lead misalignment.
Therefore, to suitably bond TAB leads to device 10 illustrated in FIG. 1A, the tail portions 17 of the ball bumps must be removed or flattened. One of the most common ways of flattening the ball bumps is by a process known as "coining". By pressing a flat surface, for example a metal plate (not shown), against the die surface, ball bumps 16 are flattened as illustrated in FIG. 2A. Although the flattened upper surface of ball bumps 16 establishes a suitable surface on which to bond TAB leads, coining has disadvantages. One disadvantage is that coining is a mechanical operation which can crack or otherwise damage the semiconductor die if not properly controlled. Furthermore, coining is an additional step in the semiconductor fabrication process which leads to an increase in manufacturing cost and ultimately an increase in device cost. Nor does coining eliminate potential misalignment problems. Bumps are arranged on the semiconductor die in a designated pattern corresponding to the bond pad locations. TAB leads which are to be bonded to these bumps are arranged in an identical pattern so that the leads will match up with the bumps during the lead bonding operation. As FIG. 2B illustrates, the slightest misplacement of bump 16 or of lead 13, as shown by a dashed lead 13'", may result in offset bonding of the leads or missed bonds altogether, thereby creating a potential for short-circuits to adjacent leads and/or open circuits. Misalignment problems may also arise if the bonding surface of the bumps is not completely level and horizontal with the die surface such that the lead slips off the bump during bonding. These misalignment problems, such as that illustrated in FIG. 2B, are not only associated with ball bumping techniques, but also with the previously described plated bumping technique.
Therefore, a need exists for an improved semiconductor device, and more specifically for a semiconductor device having TAB leads and a method for making the same which reduces fabrication costs and manufacturing time and which provides a self-aligned TAB lead to substantially reduce lead bonding misalignment.